Deep currents and the eastward salinity tongue in the equatorial Atlantic: Results from an eddy‐resolving, primitive equation model

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Abstract
The high‐resolution model of the wind‐driven and thermohaline circulation in the Atlantic Ocean developed in recent years as a “community modeling effort” for the World Ocean Circulation Experiment is examined for the temporal and spatial structure of the deep equatorial current field and its effect on the spreading of North Atlantic Deep Water (NADW). Under seasonally varying wind forcing, the model reveals a system of basin‐wide zonal currents of O(5 cm s−1), alternating east‐west, and oscillating at an annual period. The current fluctuations are induced by the seasonal cycle of the wind stress in the equatorial Atlantic and show characteristics of long equatorial Rossby waves with westward phase propagation of about 15 cm s−1. The mean flow in the deep western tropical Atlantic is governed by a deep western boundary current (DWBC) with core velocities of more than 10 cm s−1. Only a small fraction of the DWBC branches off at the equator, with correspondingly low mean eastward currents of only about 1 cm s−1. Despite this weak advection along the equator, a well‐developed salinity tongue is observed in the model, which is reminiscent of observed property distributions at the upper NADW level. The model evaluation indicates the salinity pattern to be a result of a balance between mean zonal advection and meridional diffusion of salt. The presence of the zonal current oscillations appears to have no significance for the existence of the salinity tongue.

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